J. Phys. Chem. C 2010, 114, 15021–15028
15021
Carbon Dioxide Hydrogenation on Cu Nanoparticles Ching S. Chen,* Jia H. Wu, and Tzu W. Lai Center for General Education, Chang Gung UniVersity, 259 Wen-Hwa first Road, Kwei-Shan Tao-Yuan, Taiwan, 333, Republic of China ReceiVed: May 28, 2010; ReVised Manuscript ReceiVed: August 4, 2010
The reaction mechanism and active sites for CO2 adsorption during the reverse water-gas shift (RWGS) reaction on silica-supported Cu nanoparticles were investigated. The Cu nanoparticles prepared by an atomic layer epitaxy technique were shown to strongly bind CO2 molecules, as evidenced by the two main peaks with maxima near 353 (R peak) and 525 K (β peak) observed during temperature-programmed desorption experiments. The high-temperature peak (525 K), which corresponds to CO2 in the β state, indicates that β-state CO2 was the dominant species for the RWGS reaction. The values for the activation energy of CO2 desorption were 33.4 and 74.4 kJ/mol for R-CO2 and β-CO2, respectively. The sites for CO2 adsorption were different from those for CO adsorption (defect sites and sites with highly dispersed Cu particles). It is likely that CO2 hydrogenation occurs on a low-index plane. We propose that the mechanism for the RWGS reaction mainly involves formation of formate species. 1. Introduction Carbon dioxide, which is primarily generated from combustion, is the most important and abundant greenhouse gas. It is also the main contributor to the greenhouse effect. Reducing the release of CO2 and implementing CO2 recycling techniques have therefore become increasingly important. To this end, chemical conversion of CO2 via catalytic reactions is considered the most promising process for CO2 utilization.1 Hydrogenation of CO2 is particularly attractive because it can lead to a variety of useful organic compounds, such as formic acid, methanol, carbon monoxide, methane, and hydrocarbons.2-21 Reports in the literature indicate that heterogeneous hydrogenation of CO2 usually results in CO production with high efficiency via the reverse water-gas shift (RWGS) reaction, H2 + CO2 f CO + H2O.10-21 However, CO formation is usually attributed to the first step of CO2 hydrogenation on solid metal catalysts. Thus, the RWGS reaction could be used to produce CO, which is valuable and necessary in several chemical processes, from inexpensive CO2. Because of the broad range of applications and the importance of the RWGS reaction, studies on adsorption sites, reaction mechanisms, and effects of promoters have been conducted to elucidate CO formation during a RWGS reaction.10-21 The RWGS reaction may be directly or indirectly relevant to several industrial catalytic technologies, such as methanol synthesis, methane reforming with carbon dioxide, ammonia synthesis, the Fischer-Tropsch reaction, and steam reforming of hydrocarbons.22-29 The CAMERE process (carbon dioxide hydrogenation to form methanol via a reverse water-gas shift reaction) is frequently used to produce methanol from CO2.30,31 In this process, CO2 and H2 are first converted into CO and H2O via the RGWS reaction and then H2, CO, and CO2 are used to synthesize methanol. It was shown that a proper feed ratio of H2, CO, and CO2 could enhance methanol synthesis. The CAMERE process is advantageous for methanol production due to low equipment costs and ease of operation.30 Our previous studies showed that * Corresponding author. E-mail:
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large amounts of carbon nanofibers can be synthesized from a RWGS reaction over a Ni-K/Al2O3 catalyst and that the rate of CO formation does not depend on the deposition of carbon on the catalysts.32-34 The WGS reaction (CO + H2O f H2 + CO2) has been extensively used to convert fossil fuels into hydrogen.35-39 The RWGS reaction can occur simultaneously with the WGS reaction as the two reactions can easily reach equilibrium in a reformer system. The CO/H2 ratio can be adjusted Via the RWGS reaction to achieve higher electrical efficiencies in solid oxide fuel cells (SOFCs) that operate at very high temperatures (773-1273 K).40 The equilibrium constant of the RWGS reaction can shift to higher values at elevated temperatures because it is an endothermic process (H2 + CO2 f CO + H2O, ∆H° ) +41 kJ/mol). Copper, a so-called low-temperature catalyst, is therefore usually not recommended as a catalyst for this reaction because it is thermally unstable. However, copperbased catalysts could become useful if they were thermally stable. We previously used atomic layer epitaxy (ALE) as an alternative method to prepare uniform Cu nanoparticles supported on SiO2. Nanoparticles with 0.42 dispersion (ratio of surface atoms to total atoms), an average diameter of 2.4-3.4 nm, and a narrow size distribution (
